1. Field of the Invention
The conservation of paper and other cellulosic materials has importance for libraries and for archives. Paper deteriorates mechanically primarily because of either the intrinsic acid nature of the pulp, or the introduction of acids during processing. Over time, the acid promotes hydrolysis of the cellulose, reducing its strength and causing embrittlement. The neutralization or deacidification of paper has been seen as a necessary requirement for lengthening the useful life of paper that is initially acidic.
2. Description of the Prior Art
Various methods have been proposed for the deacidification of paper. The simplest consists of the immersion of the paper in an aqueous solution of alkali, followed by drying, as described in U.S. Pat. No. 2,033,452, Schierholtz, O. J., and Barrow, W. J., "Deacidification and Lamination of Deteriorated Documents", American Archivist 28,285-290 (1965). Aqueous alkaline sprays have also been proposed by W. J. Barrow Research Laboratory, "Permanence/Durability of the Book", Dietz Press Inc., Richmond, Virginia, p.22 (1963). Both these methods suffer from the problem of requiring the handling of sheets in the wet state, with the consequent risk of damage, as well as introducing effects such as curl and cockle caused by uneven wetting and drying. To overcome this, various non-aqueous solvent treatments have been proposed. The earliest of these is a treatment with a solution of barium hydroxide in methanol developed by Baynes-Cope, "The Non Aqueous Deacidification of Documents", Restaurator 1(1) 2-9 (1969). Smith in U.S. Pat. No. 3,676,182 discloses a method of using a magnesium alkoxide in an organic solvent such as alcohol or a Freon (Trade Mark for fluorocarbons). Kelly in U.S. Pat. No. 3,939,091 discloses a method using methyl magnesium carbonate in methanol or a halogenated hydrocarbon. Kaminski and Wediger in U.S. Pat. No. 5,104,997 discloses a method using magnesium or zinc alkoxides dissolved in various hydrocarbon or halocarbon solvents. Williams and Kelly in U.S. Pat. No. 4,051,276 discloses a treatment using certain organo-metallic compounds, specifically diethyl zinc, in an organic solvent. Kundrot in U.S. Pat. No. 4,522,843 discloses a method of treatment using particles of inorganic alkaline hydroxides or carbonates dispersed in air or in Freon.
Gaseous methods have also been proposed. The simplest, neutralization with ammonia, is described by Barrow, above, and is claimed not to be effective. While a pH of an originally acid paper could be brought to neutrality using ammonia vapor, the deacidification was temporary presumably because of the volatility of ammonia and its weak alkalinity. The paper became acid again after a few weeks. Other stronger less volatile alkalis have been proposed such as morpholine disclosed by Kusterer and Sproull in U.S. Pat. No. 3,771,958. Langwell in U.S. Pat. No. 3,472,611 discloses a treatment in which a carbonate or acetate of one of the amines such as cyclohexamine is prepared and deposited on paper which is interleaved between the sheets to be treated. As the salt slowly decomposes, the cyclohexamine vapor is made available to the paper and neutralizes it over a period of weeks. It is part of the disclosure that the cyclohexamine salt does not need to be in contact with the paper to be treated since the transmission of the cyclohexamine occurs through the vapor phase.
Although some of these methods are in use, none has been widely accepted. There is uncertainty that the treatments are entirely benign especially towards the adhesives, the bindings and the printing inks. Moreover some of the treatments use chemicals that are increasingly suspect as health hazards, or as threats to the environment, as described by Smith, R. D., "Deacidification Technologies--State of the Art", in "Paper Preservation", TAPPI Press, Ed. P. Luner (1990). Finally the treatment methods are expensive, requiring specialized treatment equipment, expensive chemicals and trained operators.
It has been known for some time that free acid in paper can migrate to paper in contact with it, under air dry conditions, as described by Kozak, J. J. and Spatz, R. E., "Deacidification of Paper by the Bookkeeper Process", in "Paper Preservation", TAPPI Press, Ed. P. Luner (1990). This occurs even when the acid is non volatile, for example sulphuric acid.
The migration of ions in air-dry paper is also known from evidence of electrical conductivity. At 50% relative humidity, paper with a moisture content of about 6% can have an electrical conductivity several orders of magnitude higher than that of bone dry paper as described by Baum, G. A., "Electrical Properties: I. Theory", in "Handbook of Physical and Mechanical Testing of Paper and Paperboard", Ed. R. Mark, Marcel Dekker, New York, p. 175-178 (1984), and this is attributed to the freedom of cations such as calcium, magnesium or sodium to migrate through the anionic, water-swollen fibres.
In a sheet of mechanical or chemical pulp, deacidification cannot be achieved simply by the migration of free acid. These pulps always contain acidic groups bound within the cell walls of the pulp, with counter ions associated with them. For deacidification to be achieved, and to meet the condition of electrical neutrality, hydrogen counter-ions must be replaced by other cations such as calcium, magnesium or sodium which must migrate into the sheet.